net-sysfs: Slightly optimize 'xps_queue_show()'

The 'mask' bitmap is local to this function. So the non-atomic
'__set_bit()' can be used to save a few cycles.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>